The present invention relates to augmented reality displays and, in particular, it concerns binocular augmented reality displays with arrangements for adjusting alignment of the left-eye and right-eye displays of a binocular augmented reality display, and corresponding alignment methods.
Augmented reality spectacles must be aligned accurately in order to provide an effective binocular observation experience of the augmented image, and even relatively small misalignment may risk causing eye strain or headaches. Conventional approaches typically involve mounting the left-eye and right-eye displays on a mechanically rigid common support structure, illustrated in FIG. 1A, to achieve preliminary alignment and a fixed relative position of the displays. Final fine alignment is achieved by electronic shift of the image, as illustrated schematically in FIG. 1B, which shows an image generating matrix 30 (i.e., the physical extremities of the display field of view), and a transformed projected image 32 according to a calibration matrix, typically programmed into firmware associated with each display, to achieve correct alignment between the displays. The margins between 30 and 32 are designed into the system to accommodate any transformation required to correct misalignment within predefined limits.
An exemplary alignment process according to this approach is illustrated herein with reference to FIGS. 1A-2. The electronic alignment parameters are generated by placing the spectacles in front of two co-aligned cameras and comparing the orientation of the augmented images generated by the two projectors. The derived calibration data is introduced to the transformation firmware of the image projectors. Alternatively, the mechanical alignment of the optical system can be accurate to within the required optical accuracy. The above alignment process requires a dedicated optical alignment bench, and is only suitable for implementation in a production facility.
There is a need to implement augmented reality spectacles in a lightweight and compact form factor in order to make the technology more suitable for the consumer market. Lightweight implementations, however, often lack sufficient mechanical rigidity to ensure invariant alignment of the two displays over time, instead being subject to variations due to thermal variations and other mechanical or environmental influences.
Additionally, the inter-pupillary distance (IPD, distance between the eyes) can vary by up to 15 millimeters for different people. As a result, if the two projectors are connected rigidly, each of the eye-boxes (i.e., the illumination area of each projector where the eye pupil is expected to be, shown as region 10 in FIG. 1A) must be wider by 15/2=7.5 mm for each eye in order to accommodate every possible user having any IPD within the defined margin. The large eye-box dictates bulkier and more expensive optics. If a mechanism is provided for IPD adjustment, this typically introduces additional uncertainty into the alignment between the two displays, rendering any pre-calibrated alignment correction unreliable.